1. Field of the Invention
The present invention relates to a transmission actuator for use in a four-wheel drive mechanism or a limited slip differential mechanism for automobiles, or in general industrial machinery.
2. Description of the Related Art
A document referred to as Patent Literature No. 1 below is known to describe as a conventional transmission actuator an electric actuator in which a motor is provided externally. An actuator for friction engagement devices described in JP-B-08-019971 will be described in detail below.
FIG. 7A is a half sectional view of an actuator 50 for friction engagement devices.
As shown in FIG. 7A, a friction multi-plate clutch 65 of the actuator 50 for friction engagement devices is provided between a clutch hub 66 connected to a front wheel side hollow shaft 62 and a clutch drum 67 connected to a rear wheel side hollow shaft 61, and the torque (rotational torque) of a motor 71, which is a torque generating mechanism, is designed to be used as a pressing force for the clutch 65. Namely, a drive gear 72 on which two kinds of gears 72a, 72b are formed is mounted on a drive shaft 71a of the motor 71, and driven gears 73, 74 are disposed so as to mesh with these two gears 72a, 72b, respectively. Hollow shafts 75, 76 having respectively cam surfaces 69, 70 provided on flange portions thereof are spline connected to the gears 73, 74 via a roller 68, and furthermore, a piston 78 is provided to connect to the shaft via a bearing 77. In addition, the rear wheel side hollow shaft 61 is supported on a transfer case 81 via a bearing 80, and a transfer drive gear 63 and a cover 82 are fixed to the clutch drum 67 with a bolt, the cover 82 being supported between the hollow shafts 75, 76 and the transfer case 81 by bearings 83, 84. Note here that a shaft on a center line is an axle 60.
Next, the operation of the actuator will be described. When the motor 71 rotates, the hollow shaft 75 and the hollow shaft 76 rotate while slightly rotating relative to each other due to a slight difference in gear ratio between the gearset of the gear 72 and the gear 74 and the gearset of the gear 72 and the gear 73. Due to this, the torque is largely increased between these two shafts, and a strong thrust is generated by a motion direction changing mechanism comprising the cam surfaces 69, 70 and the roller 68, and the thrust so generated pushes against the piston 78 via the bearing 77, whereby the friction multi-plate clutch 65 is brought into engagement. A reaction force generated then is received via the bearing 77. Thus, in order to obtain a large power transmission capability with a clutch of a limited size, it is inevitable that the thrust needs to be increased, and to make this happen, there is provided a method for increase the torque of the motor or a method for increasing the gear ratio of the speed reduction mechanism.
According to Patent Literature No. 1 above, however, since an outside diameter line of the motor protrudes from the transmission case, there is caused a problem that a limitation is imposed on the mounting position of the motor. In addition, since the motor is exposed, there is also caused a problem that the motor may be damaged. Furthermore, due to structural characteristics that the motor 71 and the drive gear 72 of the speed reduction mechanism are disposed concentrically and that the outside diameter line of the motor 71 protrudes largely outwardly of the drive shaft 71a, when the diameter of the motor is enlarged with a view to obtaining a large torque, since a center distance between a driven shaft 60a and the drive shaft 71a of the motor 71 is inevitably increased, the drive gear 72 and the driven gears 73, 74 have to be enlarged, which results in increases in mass and inertia moment, leading to a problem that the response in transmitting power is deteriorated.
On the other hand, when attempting to change gear ratios with the center distance being maintained, either the drive gear 72 or the driven gears 73, 74 should be made smaller in diameter, and since the stress the tooth face has to carry is increased while the diameter of the gear is decreased, from the viewpoint of maintaining the strength of the gear the tooth thickness needs to be increased, and, as has been described above, this triggers the problem that the mass is increased to thereby deteriorate the response.